1. Field of the Invention
The present invention relates to a crystal oscillator and the manufacturing method thereof, the crystal oscillator preferred for use in computer clock generators local oscillators to, and filters for wireless communication equipment, and the like, with stable resonance frequency and a stable filter frequency being obtained even under conditions of ambient temperature fluctuation.
2. Description of the Related Art
Crystal oscillators which provide a stable resonance frequency during temperature changes have been used in the resonance circuitry of oscillators which generate electrical signals at a certain frequency.
FIG. 11 is a perspective view of a known crystal oscillator. FIG. 11 shows an AT-cut crystal substrate 1 which possesses an electrical axis (X) and wherein the temperature coefficient of the frequency is 0, electrode portions 2, 3 for excitation comprised of aluminum, gold, or the like, formed on both sides of the crystal substrate, and an excitation portion 4 which is the box-shaped area defined between the electrode portions 2 and 3.
This crystal oscillator is capable of generating electrical oscillation of a natural frequency within a range of around 1 kHz to 100 MHz, by means of applying a high-frequency voltage approximating a resonance frequency to the electrode portions 2 and 3.
Now, the resonance frequency of the above-described crystal oscillator possesses properties described by a curve of the third order in the event that the ambient temperature is employed as a parameter. Consequently, fluctuation in the resonance frequency is negligible when temperature fluctuation is small, but in the case where the temperature fluctuation is great, fluctuation in the resonance frequency becomes objectionably large.
Accordingly, temperature sensing devices which can provide a temperature compensating voltage to counteract this characteristic have been provided. An example is the Temperature Compensated X""tal Oscillator (TCXO) wherein the resonance frequency thereof is changed in a manner generally linear with the change in ambient temperature, by means of combining with a temperature compensating circuit which employs a thermistor described by an exponential function.
Further, generally, existence of a twin crystal within the crystal of the electronic device has adverse effects upon the properties of the device, and accordingly, it is considered to be imperative that there is no twin crystal formation within the afore-mentioned crystal. Also, it is well-known that there is no twin crystal formation within the crystal substrate used as the oscillator of the afore-mentioned crystal oscillator or the like.
However, there are various problems with the above-described crystal oscillator (TCXO), such as an increase in cost resulting from the additional electronic components required in forming the temperature compensating circuit in addition to the afore-mentioned thermistor; complicated procedures required for adjustment of the circuit, and the like.
On the other hand, it had been thought that the resonance frequency/temperature properties of the crystal substrate described by a curve of the third order were properties inherent to the crystal substrate, with no improvement to the crystal substrate itself possible.
Accordingly, the present invention is directed to solving the afore-mentioned problems, and the object thereof is to provide a crystal oscillator wherein a stable resonance frequency and a stable filter frequency can be obtained even under conditions of ambient temperature fluctuation, by means of a relatively simple temperature compensation circuit, wherein handling is easy and no complicated adjustment is necessary, and further wherein low costs can be realized, and also to provide a method for manufacturing such a crystal oscillator.
A crystal exhibits xcex1-xcex2 phase transition at 573xc2x0 C. (Tc), but the transition temperature decreases due to stress and the like, and it is known that the electrical axis (X-axis) is inverted at temperatures below Tc.
The inventors have discovered that applying metal electrode portions to the surface of the crystal substrate and providing heat treatment causes inversion of the axis of the crystal plate at temperatures far below Tc, depending on the direction of cut and the type of metal. The inventors thereby have provided a crystal oscillator with the crystal substrate provided with an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion, and a manufacturing method thereof.
Accordingly, the crystal oscillator according to the present invention comprises: a crystal substrate; and electrode portions for excitation formed on either face of the crystal substrate so as to form an excitation portion of the area defined between the electrode portions, wherein an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion is formed within the crystal substrate at a position other than that of the excitation portion.
Because this crystal oscillator has been provided with an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion, temperature compensation is performed by causing leakage of a portion of the oscillation energy generated within the afore-mentioned excitation portion to the afore-mentioned axis inversion portion. For example, an AT-cut crystal substrate has a resonance frequency which possesses negative temperature properties at room temperature, whereas the afore-mentioned axis inversion portion has a resonance frequency which possesses positive temperature properties. Thus, the temperature properties of the excitation portion are compensated by means of the afore-mentioned axis inversion portion. Consequently, a stable resonance frequency can be obtained even under conditions of ambient temperature fluctuation by means of a relatively simple temperature compensation circuit.
The crystal oscillator may have an axis inversion portion formed adjacent to at least one side of the excitation portion.
The crystal oscillator may have an axis inversion portion formed in the periphery of the excitation portion.
Because the axis inversion portions of these crystal oscillators are formed adjacent to at least one side of the excitation portion or in the periphery thereof, temperature compensation is corried out in a more precise manner by causing leakage of a portion of the oscillation energy generated within the afore-mentioned excitation portion to the afore-mentioned axis inversion portion.
The crystal oscillator may have electrode portions for excitation provided on the axis inversion portion so as to form a temperature sensor portion.
According to this crystal oscillator, the afore-mentioned axis inversion portion serves as a temperature sensor having temperature coefficients in a generally linear manner. Accordingly, applying this oscillator to known crystal oscillators (TCXO) permits obtaining of temperature information of the crystal substrate directly, thus enabling temperature compensation with a high degree of precision.
The method for manufacturing a crystal oscillator according to the present invention includes a crystal substrate and electrode portions for excitation formed on either face of the crystal substrate so as to form an excitation portion of the area defined between the electrode portions, the manufacturing method comprising the steps of: forming metal film on at least one side of the electrode portions for excitation formed on the surface of the crystal substrate; and subsequently subjecting the crystal substrate to thermal treatment at a temperature equal to or below the xcex1-xcex2 transition temperature of the crystal, thus forming within the crystal substrate an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion.
According to this manufacturing method, the crystal substrate upon which metal film is formed is subjected to thermal treatment at a temperature equal to or below the xcex1-xcex2 transition temperature of the crystal, thus causing inversion of the electrical axis at the position within the crystal substrate at which the inversion portion should be formed, at a temperature below the xcex1-xcex2 transition temperature, due to stress caused by the afore-mentioned metal film. Accordingly, an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion is formed within the crystal substrate at a position other than that of the afore-mentioned excitation portion, and thus a twin crystal structure including the afore-mentioned axis inversion portion and the afore-mentioned excitation portion is formed within the crystal substrate.
Consequently, an axis inversion portion possessing an electrical axis opposite to the electrical axis of the excitation portion is formed within the crystal substrate at a position other than that of the afore-mentioned excitation portion, and thus a crystal oscillator can be manufactured wherein a stable resonance frequency and a filter frequency can be obtained even under conditions of ambient temperature fluctuation.
The method for manufacturing a crystal oscillator preferably includes a metal film consisting of one of the following: Cr film, Ni film, or NiCr film.
According to this manufacturing method, inversion of the electrical axis occurs at temperatures around 540xc2x0 C. to 550xc2x0 C. Consequently, xcex1-xcex2 transition can be caused at a temperature far below the xcex1-xcex2 transition temperature, when any one of a Cr film, Ni film, or NiCr film is used.
The method for manufacturing a crystal oscillator includes a thermal treatment which is conducted within either an atmosphere of inert gas or within a vacuum.